Developing device and image forming apparatus

ABSTRACT

A developing device using a magnetic mono-component developing agent includes a rotary sleeve that accommodates a stationary magnet, and a toner layer thickness regulating member that regulates a thickness of a toner layer formed on the rotary sleeve. The toner layer thickness regulating member includes a blade formed of a plate-like member made of a magnetic material and a magnet attached to the blade on a side upstream in a rotation direction of the rotary sleeve. The magnet is disposed in such a manner that a direction of a magnetic field generated in a portion of the magnet on a side of the rotary sleeve is almost in parallel with the rotation direction of the rotary sleeve.

BACKGROUND OF THE INVENTION

1. Field of the Invention.

The present invention relates to a developing device that forms a tonerimage by supplying a magnetic developing agent toward an image carrieron which an electrostatic latent image is formed by anelectrophotographic method, and an image forming apparatus equipped withthe developing device.

2. Description of the Related Art.

A developing device applied to an image forming apparatus adopting anelectrophotographic method forms a toner image by supplying tonerparticles as a developing agent toward the peripheral surface of thephotoconductive drum on which an electrostatic latent image is formedaccording to image data. The image forming apparatus equipped with sucha developing device transfers the toner image formed on thephotoconductive drum onto a recording medium, such as a sheet. Thetransferred toner image is fixed onto the recording medium by heatingand applying a pressure to the recording medium by a fixing deviceprovided downstream of the photoconductive drum in the sheettransportation direction. By these operations, the image formingapparatus forms an image according to the image data on the recordingmedium.

The developing device is provided with a rotary sleeve to supply tonerparticles to the photoconductive drum, and a blade is provided at aposition upstream of the position at which the rotary sleeve and thephotoconductive drum oppose each other in the rotation direction of therotary sleeve. By regulating the thickness of a toner layer that isformed thick on the rotary sleeve with the blade, it is possible toprevent an excessive supply of toner particles to the photoconductivedrum. The developing device configured in this manner is therefore ableto supply toner particles in a homogeneous state to the photoconductivedrum via the rotary sleeve.

As a developing device using a magnetic mono-component developing agent(magnetic toner particles), there is known, for example, a magneticmono-component developing device that regulates the thickness of a tonerlayer made up of magnetic toner particles on the rotary sleeveaccommodating a stationary magnet using a non-contact magnetic blade. Insuch a magnetic mono-component developing device, when the toner layerformed thick on the rotary sleeve passes by the position at which therotary sleeve and the magnetic blade oppose each other (hereinafter,referred to as the layer thickness regulating position), the toner layeris divided in two between the rotary sleeve and the magnetic blade.Toner particles closer to the magnetic blade adhere to the magneticblade and toner particles closer to the rotary sleeve move away from themagnetic blade as they are attracted to the rotary sleeve by a magneticattraction force of the stationary magnet accommodated in the rotarysleeve. In this manner, the magnetic mono-component developing deviceregulates the thickness of the toner layer without the magnetic bladecontacting with the rotary sleeve instead of scarping off the tonerlayer using the blade. Accordingly, the magnetic mono-componentdeveloping device has minor damage on the magnetic toner particles, theblade, and the rotary sleeve even when high-speed printing is executed,and is therefore suitably applied to an image forming apparatus thatrequires high-speed printing and high durability.

However, because the magnetic mono-component developing device regulatesthe thickness of the toner layer with the magnetic attraction force ofthe stationary magnet accommodated in the rotary sleeve or the like, itis susceptible to the surrounding environment, such as the chargingproperty of toner particles, free fine toner particles, an externaladditive to toner particles, and the surface nature of the rotarysleeve. Accordingly, disturbance may possibly occur in the toner layerwhose layer thickness has been regulated, and the toner layer formedunder such a condition may have irregularities in density.

As a developing device that reduces the occurrence of irregularities indensity in the toner layer as above, there is known a developing deviceas described in JP-A-2003-167426 (Reference D1), in which a magnet isattached to the surface of the blade on the side upstream in therotation direction of the rotary sleeve.

The developing device described in Reference D1 is able to regulate thethickness of the toner layer without disturbing the toner layer when astrong magnetic field is generated at the end surface of the blade onthe rotary sleeve side by the magnet attached to the blade, and ahomogeneous toner layer having little irregularities in density can beobtained. However, depending on the location of the magnet attached tothe blade, there may be a case where a strong magnetic field is alsogenerated on the end surface of the magnet on the rotary sleeve side. Insuch a case, a large amount of magnetic toner particles adhere to theend surface of the magnet on the rotary sleeve side. In a case where thedeveloping device is used over a long period or at a high temperature inthis state, toner particles adhering to the magnet aggregate to formclusters. When such aggregated toner particles enter at the layerthickness regulating position, a part mixed with the aggregate tonerparticles is in a condition that the thickness and the charging state ofthe toner layer are different from those in the other parts. This givesrise to a streak-like image defect.

SUMMARY OF THE INVENTION

An object of the invention is to provide a developing device not onlycapable of forming a toner layer having little irregularities in densitybut also capable of preventing the occurrence of an image defect. Also,another object is to provide an image forming apparatus equipped withsuch a developing device.

A developing device using a magnetic mono-component developing agentaccording to one aspect of the invention to achieve the above and otherobjects includes: a rotary sleeve that accommodates a stationary magnet;and a toner layer thickness regulating member that regulates a thicknessof a toner layer formed on the rotary sleeve. The toner layer thicknessregulating member includes a blade formed of a plate-like member made ofa magnetic material, and a magnet attached to the blade on a sideupstream in a rotation direction of the rotary sleeve. The magnet isdisposed in such a manner that a direction of a magnetic field generatedin a portion of the magnet on a side of the rotary sleeve is almost inparallel with the rotation direction of the rotary sleeve.

An image forming apparatus according to another aspect of the inventionincludes an image carrier on which an electrostatic latent image is tobe formed, and a developing device that develops the electrostaticlatent image formed on the image carrier by supplying a magneticmono-component developing agent to the image carrier. The developingdevice has the configuration described above.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a view schematically showing the configuration of an imageforming apparatus (copying machine) equipped with a developing deviceaccording to one embodiment of the invention;

FIG. 2 is a schematic view showing the periphery of an image formingportion in the image forming apparatus in FIG. 1;

FIG. 3 is a cross section showing a developing unit (developing device)shown in FIG. 2;

FIG. 4 is a partially enlarged cross section showing a developing rollerand a toner layer thickness regulating member shown in FIG. 3;

FIG. 5 is a view showing the result of the analysis on magnetic fieldsin a case where magnetic poles are formed in the positional relation asspecified in FIG. 4;

FIG. 6 is a partially enlarged cross section showing a developing rollerand a toner layer thickness regulating member in a developing deviceaccording to a comparative embodiment;

FIG. 7 is a view showing the result of the analysis on magnetic fieldsin a case where magnetic poles are formed in the positional relation asspecified in FIG. 6; and

FIG. 8 is a partially enlarged cross section showing the developingroller and the toner layer thickness regulating member in the developingunits according to one embodiment of the invention and the comparativeembodiment when the magnetic flux density was measured.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

An example of an image forming apparatus equipped with a developingdevice according to one embodiment of the invention will be described. Acopying machine will be described as an example of the image formingapparatus. It should be appreciated, however, that the image formingapparatus is not limited to a copying machine, and it may be a facsimilemachine, a printer, and so forth. Also, a photoconductive drum, which isa drum-shaped photoconductor, will be described as an example of animage carrier. It should be appreciated, however, that the image carrieris not limited to a photoconductive drum, either, and the invention isalso applicable to a belt of photoconductor and a sheet ofphotoconductor.

FIG. 1 is a view schematically showing the configuration of an imageforming apparatus (copying machine) 60 equipped with a developing deviceaccording to one embodiment of the invention. The copying machine 60 isa so-called body-inside sheet discharging type copying machine,including a sheet feeing portion 200 provided at the bottom of thecopying machine main body, an image forming portion 300 provided abovethe sheet feeding portion 200, a fixing portion 400 provided closer tothe discharging side than the image forming portion 300, an imagereading portion 500 provided on the top portion of the copying machinemain body, and a sheet discharging portion 600 disposed between thecopying machine main body and the image reading portion 500. The copyingmachine main body is provided with a sheet transportation portion 100that interconnects the sheet feeding portion 200, the image formingportion 300, the fixing portion 400, and the sheet discharging portion600.

The image forming portion 300 forms a specific toner image on a sheet byan electrophotographic process, and includes a photoconductive drum 301axially supported to be rotatable, and a charging unit 302, an exposingunit 303, a developing unit (developing device) 10, a transfer unit 305,and a cleaner 306 disposed on the periphery of the photoconductive drum301 along the rotation direction A. The developing unit 10 forms a tonerimage on the surface of the photoconductive drum 301 by developing anelectrostatic latent image using toner particles.

The fixing portion 400 is disposed downstream of the image formingportion 300 in the sheet transportation direction. It nips a sheet onwhich is transferred a toner image in the image forming portion 300 andheats and applies a pressure to the sheet using a pair of rollers (aheating roller 401 and a pressure roller 402) and thereby fixes thetoner image on the sheet.

The image reading portion 500 reads image information of an original byirradiating light from an exposing lamp to the original placed on thecontact glass and introducing reflected light to a photo-electricconverting portion via a reflection mirror.

The sheet feeding portion 200 includes plural sheet feeding cassettes201, 202, and 221. Of these cassettes, the sheet feeding cassette 221serves as a bypass tray used to replenish sheets from the side surfaceof the copying machine, and it can be closed with a cover portion 222.

Sheet transportation paths 110 are connected to the respective sheetfeeding cassettes 201, 202, and 221. The sheet transportation paths 110are headed for the image forming portion 300 and further to the sheetdischarging portion 600 by way of the fixing portion 400. Thesetransportation paths 110 together form the sheet transportation portion100. A sheet after the completion of the copying operation is dischargedonto a discharge tray 610 from a discharging roller pair 605 in thesheet discharging portion

FIG. 2 is a schematic view showing the periphery of the image formingportion 300 in the image forming apparatus, such as the copying machine60 of FIG. 1. The image forming portion 300 is a portion where aspecific toner image is formed on a recording sheet 115 by theelectrophotographic process. It includes the charging unit 302, theexposing unit 303, the developing unit 10, the transfer unit 305, astatic eliminating unit 307, and the cleaner 306 provided sequentiallyon the periphery of the photoconductive drum 301 having photosensitivityalong the rotation direction A of the photoconductive drum 301. Thelocations of the static eliminating unit 307 and the cleaner 306 may beexchanged.

The charging unit 302 provides specific potential to the surface of thephotoconductive drum 301 through generation of a corona discharge. Theexposing unit 303 forms an electrostatic latent image by irradiatinglight corresponding to a desired image on the surface of thephotoconductive drum 301 to selectively attenuate the surface potential.The developing unit 10 forms a toner image by developing theelectrostatic latent image formed on the surface of the photoconductivedrum 301 with toner particles. The transfer unit 305 transfers the tonerimage formed on the photoconductive drum 301 onto a recording sheet 115.The static eliminating unit 307 eliminates surface charges on thephotoconductive drum 301 using lamp light. The cleaner 306 is formed ofa fur brush 316 and a rubber blade 326, and removes toner particles andadditives thereof remaining on the surface of the photoconductive drum301. The cleaner 306 shown in the drawing as an example has both the furbrush 316 and the rubber blade 326. It should be noted, however, thatthere is a cleaner having only one of these components.

Heat and a pressure are applied to the recording sheet 115 on which istransferred the toner image in the image forming portion 300 by thefixing portion 400 (the heating roller 401 and the pressure roller 402),and the toner image is fixed thereon. Subsequently, the recording sheet115 is discharged onto the sheet discharge tray 610 by the sheetdischarging roller pair 605 shown in FIG. 1.

Hereinafter, the developing unit (developing device) 10 according to oneembodiment of the invention used in the copying machine will bedescribed.

FIG. 3 is a cross section showing the developing unit (developingdevice) 10 shown in FIG. 2 together with the photoconductive drum 301.An unillustrated magnetic mono-component developing agent (tonerparticles) is accommodated in toner storing portions 354 of thedeveloping unit 10, and two stirring rollers 314 and 324 that stir tonerparticles and a developing roller 14 that forces toner particles tomigrate onto the surface of the photoconductive drum 301 are provided.

The developing roller 14 is formed of a cylindrical rotary sleeve 13made of a non-magnetic material, such as aluminum, and a stationarymagnet 15 accommodated in the rotary sleeve 13. It is configured in sucha manner that the rotary sleeve 13 rotates about the stationary magnet15 while the position thereof is fixed. In addition, a toner layerthickness regulating member 16 is provided oppositely to the rotarysleeve 13 of the developing roller 14.

The stationary magnet 15 is a magnet of the 6-pole structure with themagnetic poles disposed as follows. An S pole (developing pole S1) isdisposed at a position opposing the photoconductive drum 301 and anotherS pole (blade pole S3) is disposed at a position opposing the tonerlayer thickness regulating member 16. An N pole (N1), an S pole (S2),and an N pole (N2) are sequentially disposed downstream of thedeveloping pole S1 in the rotation direction B of the rotary sleeve 13.Further, an N pole (N3) is disposed downstream of the blade pole S3 inthe rotation direction B of the rotary sleeve 13.

The toner layer thickness regulating member 16 is formed of a blade 11and a magnet 12. The blade 11 is disposed in a direction perpendicularto the rotation direction B of the rotary sleeve 13, and the magnet 12is attached to the surface of the blade 11 on the side upstream in therotation direction B of the rotary sleeve 13.

FIG. 4 is a partially enlarged cross section showing the developingroller 14 and the toner layer thickness regulating member 16 shown inFIG. 3. The blade 11 is formed of a plate-like member made of a magneticmaterial. Examples of the plate-like member made of a magnetic materialinclude but not limited to SUS340 and SUS430, and SUS430 is usedpreferably. The blade 11 has an end surface 11 a opposing the rotarysleeve 13. The end surface 11 a is also referred to as the end face 11 aof the blade 11 on the rotary sleeve side herein. The end surface 11 aof the blade 11 on the rotary sleeve side is provided to extend inparallel with the axial direction of the rotary sleeve 13 while beingspaced apart from the rotary sleeve 13 by a specific interval W1. Thespecific interval W1 is preferably 0.2 to 0.4 mm, and for example, 0.3mm. When W1 is too narrow, there is a tendency that toner particlesreadily clog in a space between the blade 11 and the rotary sleeve 13.Conversely, when W1 is too wide, there is a tendency that irregularitiesin density readily occur in the toner layer. The thickness T1 of theblade 11 can be any thickness as long as it is sufficient to firmlymaintain the position of the end surface 11 a on the rotary sleeve sidewithout bending, and for example, 2 mm. In addition, the magnet 12 hasan end surface 12 a opposing the rotary sleeve 13. The end surface 12 ais also referred to as the end face 12 a of the magnet 12 on the rotarysleeve side herein. It is preferable that the end surface 12 a of themagnet 12 on the rotary sleeve side is at a position farther from therotary sleeve 13 than the end surface 11 a of the blade 11 on the rotarysleeve side. A distance between the end surface 11 a of the blade 11 onthe rotary sleeve side and the end surface 12 a of the magnet 12 on therotary sleeve side, that is, a protrusion length W2 of the blade 11 withrespect to the magnet 12 is preferably 0.3 to 0.8 mm, and for example,0.5 mm. When W2 is too narrow, there is a tendency that toner particlesreadily clog in a space between the blade 11 and the rotary sleeve 13 ina case where a small amount of toner particles adhere to the end surface12 a of the magnet 12 on the rotary sleeve side. Conversely, when W2 istoo wide, there is a tendency that the magnet 12 can no longer bedisposed in the manner as described below (disposed so that thedirection C of the magnetic field is almost in parallel with therotation direction B of the rotary sleeve 13).

The magnet 12 is a plate-like magnet disposed in parallel with the planedirection of the blade 11. It is sufficient for the magnet 12 togenerate a specific magnetic field, and the thickness T2 thereof is, forexample, 4 mm, the length L thereof is, for example, 5 mm, and thelength in parallel with the shaft center direction of the developingroller 14 is, for example, 218 mm. The magnet 12 has a magnetizationdirection perpendicular to the plane direction of the blade 11. To bemore concrete, the lamination surface side in contact with the blade 11serves as an N pole, which has a polarity different from that of themagnetic pole (blade pole S3) formed in a portion of the stationarymagnet 15 included in the developing roller 14 so as to oppose the blade11, whereas the open surface side not in contact with the blade 11serves as an S pole, which has the polarity same as that of the bladepole S3. In other words, in this embodiment, the stationary magnet 15included in the developing roller 14 has a first magnetic pole (S pole)disposed at a position opposing the toner layer thickness regulatingmember 16, whereas the magnet 12 has a second magnetic pole (N pole)formed on the lamination surface side in contact with the blade 11 tohave a polarity different from that of the first magnetic pole andanother magnetic pole (S pole) formed on the open surface side not incontact with the blade 11 to have the same polarity as the firstmagnetic pole. In other words, the magnet 12 is disposed in such amanner that the direction (a direction in which the magnetic field lineis oriented) C of the magnetic field generated in a portion of themagnet 12 on the rotary sleeve side is almost in parallel with therotation direction B of the rotary sleeve 13 (a direction in which theouter surface of the rotary sleeve 13 moves).

FIG. 5 is a view showing the result of the analysis on the magneticfields when magnetic poles are formed in the positional relation asspecified in FIG. 4. The magnetic fields were analyzed by making adistance between the magnet 12 and the stationary magnet 15 shorter thanthe actual distance in order to make a difference from a comparativeembodiment described below easy to understand by indicating thedirections of the magnetic fields clearly. As can be understood fromFIG. 5, the magnet 12 disposed in the positional relation as specifiedin FIG. 4 is disposed in such a manner that the direction C of themagnetic field generated in a portion of the magnet 12 on the rotarysleeve side is almost in parallel with the rotation direction B of therotary sleeve 13.

It is sufficient to dispose the magnet 12 so that the direction C of themagnetic field is almost in parallel with the rotation direction B ofthe rotary sleeve 13 as described above, and the magnet 12 does notnecessarily have to be disposed according to the positional relation asspecified in FIG. 4.

In the developing unit 10, a strong magnetic field is generated at thelayer thickness regulating position by the S3 pole in the stationarymagnet 15 and the S pole on the end surface 12 c of the magnet 12 on theopen surface side. In addition, as can be understood from the fact thatthe direction C of the magnetic field as described above is formed onthe end surface 11 a of the blade 11 on the rotary sleeve side, a strongN pole is induced. Further, because a magnetic field is generated in aportion of the magnet 12 on the rotary sleeve side in such a manner thatthe direction C of the magnetic field is almost in parallel with therotation direction B of the rotary sleeve 13, a strong magnetic pole isnot formed on the end surface 12 a of the magnet 12 on the rotary sleeveside.

Hereinafter, operations of the developing unit 10 will be described.

Toner particles inside the toner storing portions 354 in the developingunit 10 are attracted by magnetic forces of the N2 pole and the N3 polein the stationary magnet 15 and transported to the layer thicknessregulating position while being moved upward by rotations of the rotarysleeve 13. In this instance, because a strong magnetic field has beengenerated between the blade 11 and the rotary sleeve 13 by the N pole onthe end surface 11 a of the blade 11 on the rotary sleeve side and theS3 pole in the stationary magnet 15, the toner layer passes by the layerthickness regulating position in an almost homogeneous state whereirregularities in density are reduced to an extremely low extent. Whenthe toner layer moves away from the layer thickness regulating position,toner particles closer to the blade 11 adhere to the blade 11 owing tothe N pole on the end surface 11 a of the blade 11 on the rotary sleeveside. Meanwhile, toner particles closer to the rotary sleeve 13 areattracted toward the rotary sleeve 13 by the S3 pole in the stationarymagnet 15 and move away from the blade 11. In this manner, the tonerlayer on the rotary sleeve 13 is divided in two in a homogeneous stateso that the layer thickness is regulated. Thereafter, the toner thinlayer whose layer thickness is regulated and made thin is headed for thephotoconductive drum 301.

As has been described, in this embodiment, because a strong N pole isinduced on the end surface 11 a of the blade 11 on the rotary sleeveside by the magnet 12 in the toner layer thickness regulating member 16,a strong magnetic force acts between the blade 11 and the rotary sleeve13. This makes the toner layer at the layer thickness regulatingposition least susceptible to the environment other than this strongmagnetic force. Consequently, disturbance in the toner layer at thelayer thickness regulating position is eliminated. It is thus possibleto form an almost homogeneous toner layer having little irregularitiesin density even when the layer thickness is regulated.

Further, because a strong magnetic pole is not formed on the end surface12 a of the magnet 12 on the rotary sleeve side, toner particles hardlyadhere to the end surface 12 a of the magnet 12 on the rotary sleeveside. It is thus possible to prevent the occurrence of an image defectresulting from aggregated toner particles.

As has been described, the developing device 10 configured as above isnot only capable of forming the toner layer having little irregularitiesin density, but is also capable of preventing the occurrence of an imagedefect.

A developing device 20 in which the magnet 12 is located differentlyfrom the invention will now be described as a comparative embodiment forthe purpose of comparison with the invention. Because the developingdevice 20 is the same as the developing device 10 of the inventionexcept for the location of the magnet 12, like components are labeledwith like reference numerals with respect to FIG. 4 and descriptionswill not be repeated herein.

FIG. 6 is a partially enlarged cross section showing the developingroller 14 and the toner layer thickness regulating member 16 in thedeveloping device 20. A magnet 17 attached to the blade 11 has an Spole, which has a polarity same as that of the magnetic pole (blade poleS3) formed in a portion of the stationary magnet 15 included in thedeveloping roller 14 so as to oppose the blade 11, on the end surface 17a on the rotary sleeve side and an N pole on the end surface 17 b on theopposite side (the side farther from the rotary sleeve 13). The magnet17 is therefore located in such a manner that the direction D of themagnetic field generated in a portion of the magnet 17 on the rotarysleeve side is almost perpendicular to the rotation direction B of therotary sleeve 13.

FIG. 7 is a view showing the result of the analysis on the magneticfields when the magnetic poles are formed in the positional relationspecified in FIG. 6. As with FIG. 5, the magnetic fields were analyzedby making a distance between the magnet 17 and the stationary magnet 15shorter than the actual distance. As can be understood from FIG. 7, themagnet 17 disposed in the positional relation as specified in FIG. 6 isdisposed in such a manner that the direction D of the magnetic fieldgenerated in a portion of the magnet 17 on the rotary sleeve side isalmost perpendicular to the rotation direction B of the rotary sleeve13.

In the developing unit 20, a strong magnetic field is generated at thelayer thickness regulating position by the S3 pole in the stationarymagnet 15 and the S pole on the end surface 17 a of the magnet 17 on therotary sleeve side. In addition, as can be understood from the fact thatthe direction D of the magnetic field as described above is formed onthe end surface 11 a of the blade 11 on the rotary sleeve side, a strongN pole is induced. Because a magnetic field is generated in a portion ofthe magnet 17 on the rotary sleeve side in such a manner that thedirection D of the magnetic field is almost perpendicular to therotation direction B of the rotary sleeve 13, a strong magnetic pole isformed on the end surface 17 a of the magnet 17 on the rotary sleeveside, which is a significant difference from the developing unit 10described above.

In the developing unit 20, because a strong N pole is induced on the endsurface 11 a of the blade 11 on the rotary sleeve side, as with thedeveloping unit 10, it is possible to form a toner layer having littleirregularities in density. However, because a strong magnetic pole isformed on the end surface 17 a of the magnet 17 on the rotary sleeveside, a large amount of toner particles adhere to the end surface 17 aof the magnet 17 on the rotary sleeve side. Accordingly, in a case wherethe developing device 20 is used over a long period or at a hightemperature, toner particles adhering to the magnet 17 may possiblyaggregate to form clusters. When aggregated toner particles enter at thelayer thickness regulating position, a part mixed with aggregated tonerparticles is in a condition that the layer thickness and the chargingstate of the toner layer are different from those in the other parts,which gives rise to the occurrence of a streak-like image defect.

Hereinafter, the magnetic flux density actually measured in thedeveloping unit 10 according to one embodiment of the invention and inthe developing unit 20 according to the comparative embodiment for thepurpose of comparison with the invention will be described.

FIG. 8 is a partially enlarged cross section showing the developingroller 14 and the toner layer thickness regulating member 16 in thedeveloping units 10 and 20 in which the magnetic flux density wasmeasured. The dimensions and the positional relations of the respectivemembers in the developing units 10 and 20 in which the magnetic fluxdensity was actually measured were as follows. The interval W1 betweenthe end surface 11 a of the blade 11 on the rotary sleeve side and therotary sleeve 13 was 0.3 mm. The thickness T1 of the blade 11 was 2 mm.The protrusion length W2 of the blade 11 with respect to the magnet 12or 17 was 0.5 mm. The magnets 12 and 17 had the thickness T2 of 4 mm andthe length L of 5 mm, and the length in a direction in parallel with theshaft center direction of the developing roller 14 was 218 mm. Thediameter R of the rotary sleeve 13 was 16 mm. The angle a between theplane direction of the blade 11 and the longitudinal direction of thestationary magnet 15 opposing the blade 11 was 4 degrees. The developingunit 10 and 20 employed a magnet of a 70 mT as the magnet 12, a magnetof a 70 mT as the magnet 17 and a magnet of 90 mT as the stationarymagnet 15, and the magnetic flux density was measured under themeasurement conditions as specified above (shapes and the positionalrelations). The magnetic flux density was measured using a Tesla meter.

In the developing unit 10 according to one embodiment of the invention,an N pole of 40 mT was formed on the edge 11 c of the end surface 11 aof the blade 11 on the rotary sleeve side upstream in the rotationdirection of the rotary sleeve 13. An S pole of 70 mT was formed in thecenter portion on the end surface 12 c of the magnet 12 on the opensurface side. An S pole of 30 mT was formed in the center portion on theS pole side on the end surface 12 a of the magnet 12 on the rotarysleeve side. Accordingly, it is understood that a strong magnetic polewas formed on the end surface 11 a of the blade 11 on the rotary sleeveside but a strong magnetic pole was not formed on the end surface 12 aof the magnet 12 on the rotary sleeve side in the developing unit 10.

In the developing unit 20 according to the comparative embodiment, an Npole of 40 mT was formed on the edge 11 c of the end surface 11 a of theblade 11 on the rotary sleeve side upstream in the rotation direction ofthe rotary sleeve 13. An N pole of 20 mT was formed in the centerportion on the N pole side on the end surface 17 d of the magnet 17 onthe open surface side, and an S pole of 20 mT was formed in the centerportion on the S pole side on the end surface 17 c on the open surfaceside. An S pole of 70 mT was formed in the center portion on the endsurface 17 a of the magnet 17 on the rotary sleeve side. Accordingly, itis understood that a strong magnetic pole was formed on the end surface11 a of the blade 11 on the rotary sleeve side, and at the same time, astrong magnetic pole was also formed on the end surface 17 a of themagnet 17 on the rotary sleeve side in the developing unit 20.

In view of the foregoing, in comparison with the developing unit 20 ofthe comparison embodiment, the developing device (developing unit) 10 ofthis embodiment is capable of maintaining almost the same magnetic forceon the end surface 11 a of the blade 11 on the rotary sleeve side whilesuppressing the magnetic force on the end surface 12 a of the magnet 12on the rotary sleeve side. Accordingly, by the operations as describedabove, the developing device (developing unit) 10 of this embodiment isnot only capable of forming a toner layer having little irregularitiesin density, but is also capable of preventing the occurrence of an imagedefect, in particular, a streak-like image defect appearing in ahalf-tone image by reducing an amount of toner particles adhering to theend surface 12 a of the magnet 12 on the rotary sleeve side.

The embodiment above described a magnetic mono-component developingdevice using a magnetic mono-component magnetic agent (magnetic tonerparticles). It should be appreciated, however, that the invention is notlimited to this configuration, and similar advantages can be achieved aslong as a developing device uses a magnetic developing agent. Examplesof a magnetic developing agent include but not limited to atwo-component developing agent containing magnetic toner particles and anon-magnetic carrier, a two-component developing agent containingnon-magnetic toner particles and a magnetic carrier, and a two-componentdeveloping agent containing magnetic toner particles and a magneticcarrier.

The specific embodiment described above chiefly includes inventionshaving the following configurations.

A developing device using a magnetic mono-component developing agentaccording to one aspect of the invention includes a rotary sleeve thataccommodates a stationary magnet, and a toner layer thickness regulatingmember that regulates a thickness of a toner layer formed on the rotarysleeve. The toner layer thickness regulating member includes a bladeformed of a plate-like member made of a magnetic material, and a magnetattached to the blade on a side upstream in a rotation direction of therotary sleeve. The magnet is disposed in such a manner that a directionof a magnetic field generated in a portion of the magnet on a side ofthe rotary sleeve is almost in parallel with the rotation direction ofthe rotary sleeve.

According to this configuration, it is possible to regulate thethickness of the toner layer formed thick on the rotary sleeve using thetoner layer thickness regulating member, which allows the tonerparticles to be supplied to the image carrier from the toner layer whoselayer thickness has been regulated.

In addition, because the magnet is attached to the blade formed of aplate-shaped member made of a magnetic material on the side upstream inthe rotation direction of the rotary sleeve, the toner layer thicknessregulating member is able to generate a strong magnetic field on the endsurface of the blade on the rotary sleeve side. Accordingly, a strongmagnetic force acts between the blade and the rotary sleeve, which makesthe toner layer at the layer thickness regulating position leastsusceptible to the environment other than this magnetic force.Consequently, disturbance in the toner layer hardly occurs at the layerthickness regulating position. It is thus possible to form a toner layerhaving little irregularities in density even when the layer thickness isregulated.

In addition, the magnet is disposed in such manner that the direction ofthe magnetic field generated in the portion on the rotary sleeve side isalmost in parallel with the rotation direction of the rotary sleeve.Hence, in comparison with a case where the magnet is disposed in such amanner that the direction of the magnetic field is perpendicular to therotation direction of the rotary sleeve, even when the magnetic fieldgenerated on the end surface of the blade on the rotary sleeve side hasabout the same strength, the magnetic field generated on the end surfaceof the magnet on the rotary sleeve side is weak. Consequently, tonerparticles hardly adhere to the end surface of the magnet on the rotarysleeve side. It is thus possible to prevent the occurrence of an imagedefect resulting from aggregated toner particles.

As has been described, the developing device of the invention is notonly capable of forming the toner layer having little irregularities indensity, but is also capable of preventing the occurrence of an imagedefect.

In the configuration described above, to be more concrete, it ispreferable that: the blade is disposed in a direction perpendicular tothe rotation direction of the rotary sleeve; the magnet is a plate-likemagnet to be disposed in parallel with a plane direction of the blade;and the plate-like magnet has a magnetization direction perpendicular tothe plane direction of the blade.

According to this configuration, it is possible to dispose the magnetwith ease in such a manner that the direction of the magnetic fieldgenerated in the portion of the magnet on the rotary sleeve side isalmost in parallel with the rotary direction of the rotary sleeve.Hence, not only is it possible to form a toner layer whose layerthickness is regulated so as to have little irregularities in density,but it is also possible to prevent the occurrence of an image defectresulting from aggregated toner particles.

In the configuration described above, it is preferable that: the magnethas an end surface opposing the rotary sleeve; the blade has an endsurface opposing the rotary sleeve; and the end surface of the magnet isat position farther from the rotary sleeve than the end surface of theblade.

According to this configuration, even when a small amount of tonerparticles adhere to the end surface of the magnet on the rotary sleeveside, toner particles hardly clog in a space between the blade and therotary sleeve. It is thus possible to prevent the occurrence of an imagedefect resulting from the clogging of toner particles.

In the configuration described above, it is preferable that: thestationary magnet has a first magnetic pole disposed at a positionopposing the toner layer thickness regulating member; and that themagnet has a second magnetic pole having a polarity different from apolarity of the first magnetic pole on a lamination surface side incontact with the blade and a magnetic pole having a polarity same as thepolarity of the first magnetic pole on an open surface side not incontact with the blade.

According to this configuration, it is possible to dispose the magnetwith more ease in such a manner that the direction of the magnetic fieldgenerated in the portion of the magnet on the rotary sleeve side isalmost in parallel with the rotation direction of the rotary sleeve.

In the configuration described above, it is preferable that a distancebetween the rotary sleeve and the blade is 0.2 to 0.4 mm. According tothis configuration, toner particles hardly clog in a space between theblade and the rotary sleeve, which makes it possible to form a suitabletoner layer.

In the configuration described above, it is preferable that a length ofthe blade protruding with respect to the magnet is 0.3 to 0.8 mm.According to this configuration, even when a small amount of tonerparticles adhere to the end surface of the magnet on the rotary sleeveside, toner particles hardly clog in a space between the blade and therotary sleeve. It is thus possible to prevent the occurrence of an imagedefect resulting from the clogging of toner particles.

Also, a developing device using a magnetic developing agent according toanother aspect of the invention includes a rotary sleeve thataccommodates a stationary magnet, and a developing agent layer thicknessregulating member that regulates a thickness of a developing agent layerformed on the rotary sleeve. The developing agent layer thicknessregulating member includes a blade formed of a plate-like member made ofa magnetic material, and a magnet attached to the blade on a sideupstream in a rotation direction of the rotary sleeve. The magnet isdisposed in such a manner that a direction of a magnetic field generatedin a portion of the magnet on a side of the rotary sleeve is almost inparallel with the rotation direction of the rotary sleeve.

In the configuration described above, to be more concrete, it ispreferable that the blade is disposed in a direction perpendicular tothe rotation direction of the rotary sleeve; the magnet is a plate-likemagnet to be disposed in parallel with a plane direction of the blade;and the plate-like magnet has a magnetization direction perpendicular tothe plane direction of the blade.

In the configuration described above, it is preferable that: the magnethas an end surface opposing the rotary sleeve; the blade has end surfaceopposing the rotary sleeve; and the end surface of the magnet is at aposition farther from the rotary sleeve than an end surface of theblade.

In the configuration described above, it is preferable that: thestationary magnet has a first magnetic pole disposed at a positionopposing the developing agent layer thickness regulating member; andthat the magnet has a second magnetic pole having a polarity differentfrom a polarity of the first magnetic pole on a lamination surface sidein contact with the blade and a magnetic pole having a polarity same asthe polarity of the first magnetic pole on an open surface side not incontact with the blade.

In the configuration described above, it is preferable that a distancebetween the rotary sleeve and the blade is 0.2 to 0.4 mm.

In the configuration described above, it is preferable that a length ofthe blade protruding with respect to the magnet is 0.3 to 0.8 mm.

Further, an image forming apparatus according to still another aspect ofthe invention includes an image carrier on which an electrostatic latentimage is to be formed, and a developing device that develops theelectrostatic latent image formed on the image carrier by supplying amagnetic mono-component developing agent to the image carrier. Thedeveloping device includes a rotary sleeve that accommodates astationary magnet, and a toner layer thickness regulating member thatregulates a thickness of a toner layer formed on the rotary sleeve. Thetoner layer thickness regulating member includes a blade formed of aplate-like member made of a magnetic material, and a magnet attached tothe blade on a side upstream in a rotation direction of the rotarysleeve. The magnet is disposed in such a manner that a direction of amagnetic field generated in a portion of the magnet on a side of therotary sleeve is almost in parallel with the rotation direction of therotary sleeve.

According to this configuration, because the developing device asdescribed above is incorporated into the image forming apparatus, thedeveloping device is not only capable of forming a toner layer havinglittle irregularities in density, but is also capable of preventing theoccurrence of an image defect. the image forming apparatus is therebyable to form a high-quality image on a recording medium.

This application is based on patent application No. 2007-021011 filed inJapan, the contents of which are hereby incorporated by references.

As this invention may be embodied in several forms without departingfrom the spirit of essential characteristics thereof, the presentembodiment is therefore illustrative and not restrictive, since thescope of the invention is defined by the appended claims rather than bythe description preceding them, and all changes that fall within metesand bounds of the claims, or equivalence of such metes and bounds aretherefore intended to embraced by the claims.

1. A developing device using a magnetic mono-component developing agent,comprising: a rotary sleeve that accommodates a stationary magnet; and atoner layer thickness regulating member that regulates a thickness of atoner layer formed on the rotary sleeve, wherein the toner layerthickness regulating member includes: a blade formed of a plate-likemember made of a magnetic material; and a magnet attached to the bladeon a side upstream in a rotation direction of the rotary sleeve, andwherein the magnet is disposed in such a manner that a direction of amagnetic field generated in a portion of the magnet on a side of therotary sleeve is almost in parallel with the rotation direction of therotary sleeve.
 2. The developing device according to claim 1, wherein:the blade is disposed in a direction perpendicular to the rotationdirection of the rotary sleeve; the magnet is a plate-like magnet to bedisposed in parallel with a plane direction of the blade; and theplate-like magnet has a magnetization direction perpendicular to theplane direction of the blade.
 3. The developing device according toclaim 1, wherein: the magnet has an end surface opposing the rotarysleeve; the blade has an end surface opposing the rotary sleeve; and theend surface of the magnet is at position farther from the rotary sleevethan the end surface of the blade.
 4. The developing device according toclaim 1, wherein: the stationary magnet has a first magnetic poledisposed at a position opposing the toner layer thickness regulatingmember; and the magnet has a second magnetic pole having a polaritydifferent from a polarity of the first magnetic pole on a laminationsurface side in contact with the blade and a magnetic pole having apolarity same as the polarity of the first magnetic pole on an opensurface side not in contact with the blade.
 5. The developing deviceaccording to claim 1, wherein: a distance between the rotary sleeve andthe blade is 0.2 to 0.4 mm.
 6. The developing device according to claim1, wherein: a length of the blade protruding with respect to the magnetis 0.3 to 0.8 mm.
 7. A developing device using a magnetic developingagent, comprising: a rotary sleeve that accommodates a stationarymagnet; and a developing agent layer thickness regulating member thatregulates a thickness of a developing agent layer formed on the rotarysleeve, wherein the developing agent layer thickness regulating memberincludes: a blade formed of a plate-like member made of a magneticmaterial; and a magnet attached to the blade on a side upstream in arotation direction of the rotary sleeve, and wherein the magnet isdisposed in such a manner that a direction of a magnetic field generatedin a portion of the magnet on a side of the rotary sleeve is almost inparallel with the rotation direction of the rotary sleeve.
 8. Thedeveloping device according to claim 7, wherein: the blade is disposedin a direction perpendicular to the rotation direction of the rotarysleeve; the magnet is a plate-like magnet to be disposed in parallelwith a plane direction of the blade; and the plate-like magnet has amagnetization direction perpendicular to the plane direction of theblade.
 9. The developing device according to claim 7, wherein: themagnet has an end surface opposing the rotary sleeve; the blade has anend surface opposing the rotary sleeve; and the end surface of themagnet is at position farther from the rotary sleeve than the endsurface of the blade.
 10. The developing device according to claim 7,wherein: the stationary magnet has a first magnetic pole disposed at aposition opposing the developing agent layer thickness regulatingmember; and the magnet has a second magnetic pole having a polaritydifferent from a polarity of the first magnetic pole on a laminationsurface side in contact with the blade and a magnetic pole having apolarity same as the polarity of the first magnetic pole on an opensurface side not in contact with the blade.
 11. The developing deviceaccording to claim 7, wherein: a distance between the rotary sleeve andthe blade is 0.2 to 0.4 mm.
 12. The developing device according to claim7, wherein: a length of the blade protruding with respect to the magnetis 0.3 to 0.8 mm.
 13. An image forming apparatus, comprising: an imagecarrier on which an electrostatic latent image is to be formed; and adeveloping device that develops the electrostatic latent image formed onthe image carrier by supplying a magnetic mono-component developingagent to the image carrier, wherein the developing device includes: arotary sleeve that accommodates a stationary magnet; and a toner layerthickness regulating member that regulates a thickness of a toner layerformed on the rotary sleeve, wherein the toner layer thicknessregulating member includes: a blade formed of a plate-like member madeof a magnetic material; and a magnet attached to the blade on a sideupstream in a rotation direction of the rotary sleeve, and wherein themagnet is disposed in such a manner that a direction of a magnetic fieldgenerated in a portion of the magnet on a side of the rotary sleeve isalmost in parallel with the rotation direction of the rotary sleeve. 14.The image forming apparatus according to claim 13, wherein: the blade isdisposed in a direction perpendicular to the rotation direction of therotary sleeve; the magnet is a plate-like magnet to be disposed inparallel with a plane direction of the blade; and the plate-like magnethas a magnetization direction perpendicular to the plane direction ofthe blade.
 15. The image forming apparatus according to claim 13,wherein: the magnet has an end surface opposing the rotary sleeve; theblade has an end surface opposing the rotary sleeve; and the end surfaceof the magnet is at position farther from the rotary sleeve than the endsurface of the blade.
 16. The image forming apparatus according to claim13, wherein: the stationary magnet has a first magnetic pole disposed ata position opposing the toner layer thickness regulating member; and themagnet has a second magnetic pole having a polarity different from apolarity of the first magnetic pole on a lamination surface side incontact with the blade and a magnetic pole having a polarity same as thepolarity of the first magnetic pole on an open surface side not incontact with the blade.
 17. The image forming apparatus according toclaim 13, wherein: a distance between the rotary sleeve and the blade is0.2 to 0.4 mm.
 18. The image forming apparatus according to claim 13,wherein: a length of the blade protruding with respect to the magnet is0.3 to 0.8 mm.